This invention relates in general to control circuits for hydraulic systems and, in particular, to a control circuit for controlling a hydraulic vehicular suspension system.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a control circuit for a hydraulic vehicle suspension system used on an articulated earthmoving scraper for minimizing the bouncing, pitching, and rolling motions encountered during vehicle operation.
Articulated wheel scrapers comprise, typically, a single axle tractor connected by a pivotal yoke mounting or hitch to a single axle trailer having a scraper bowl portion. Such vehicles operate in one of three modes; loading, transporting and unloading. As will be hereinafter discussed in detail, the vehicle suspension system must satisfy different requirements of each of these operating modes. One suitable hydraulic suspension system for articulated scrapers has been described in the inventor's co-pending patent application, Ser. No. 883,804, filed Mar. 6, 1978 now U.S. Pat. No. 4,201,273, the disclosure of which is incorporated herein by reference.
In many applications, articulted scrapers transport heavy loads of material over irregular terrain at relatively high speeds. During the transporting operation, shock forces are transmitted through the tires to the vehicle frame and, in turn, to the vehicle operator. Since transmission of such shock forces is discomforting and potentially dangerous to the machine operator, it is desirable to dampen these forces by means of a suitable suspension system. In addition, when the vehicle is loading, the loading action is improved by insuring that the driving wheels remain in a positive driving traction with the vehicle supporting surface. To this end it is desirable to permit a certain controlled degree of relative rotation, generally referred to as roll or tilt, of the tractor and trailer relative to each other about an axis extending longitudinally through the vehicle and generally parallel to the vehicle support surface. Since the resilient suspension system is most needed at higher speeds, the loading action is also improved by locking out the suspension system during loading operation. The shock absorbing and cushioning members of the suspension system are hydraulically maintained inoperative and fluctuations in the relative position of the trailer bowl and its forward cutting edge are minimized.
Another problem encountered as the vehicle is being loaded and unloaded, results from the constantly changing volume and weight of materials contained in the trailer as it is being received into or discharged from the scraper bowl. This continuous weight change causes the level of the scraper bowl to vary relative to the tractor and the vehicle supporting surface. As the weight of the material contained in the scraper bowl increases during the loading operation, the bowl tends to sink. Conversely, as the loaded material is discharged during the unloading operation, the weight of the load constantly decreases and the scraper bowl rises. The hydraulic forces supporting the vehicle raise the supported elements upwardly away from the vehicle supporting surface. The resulting cut, or the distribution of the material in the unloading operation at the dump site, will be uneven due to the cushioning and viscous damping characteristics of the vehicle suspension system. Therefore, it is desirable to maintain the bowl level within a predetermined range of movement relative to the tractor and supporting surface during loading and unloading operation.
U.S. Pat. No. 3,643,970 discloses a suspension system which locks out the vehicle suspension system while on a job site and releases it to increase stability for road travel. However, the system disclosed therein does not provide for damping the pitch or roll between the tractor and trailer members.
Systems such as disclosed in U.S. Pat. No. 3,869,141 have attempted to prevent the deterioration of vehicle driveability through controlling hydraulic cylinders operatively connected between the axle and the vehicle frame. Such systems increase resistance to rolling and pitching during high speed turns, acceleration and deacceleration, by controlling the volume of pressurizing gas or isolating the hydraulic fluid within the suspension struts by means of the horizontal forces acting on a weight suspended from a control valve. However, such a system does not compensate for changes in the vehicle position relative to the vehicle supporting surface, caused by the changing weight of the work material being received into or unloaded from the vehicle.
Another attempt to solve this problem is disclosed in U.S. Pat. No. 3,953,040 which describes a suspension system for wheel scrapers which dampens the shock and bounce motions encountered during the transporting operation, and has a lockout feature for locking out the vehicle suspension during loading. However, this system provides only limited control of the rolling or tilting motion between the tractor and trailer portions of the vehicle.